1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method used for ophthalmologic consultations.
2. Description of the Related Art
Ophthalmic examinations are widely performed for the purpose of early diagnosis of lifestyle-related diseases and diseases that rank highly among causes of loss of eyesight. A scanning laser ophthalmoscope (SLO), which is an image processing apparatus that uses the principle of a confocal laser microscope, is an apparatus that performs Raster scanning of an eye fundus using a laser that is a measuring beam and obtains a planar image at a high resolution and a high speed based on the intensity of the return light. The apparatus that captures this planar image will be referred to as an SLO apparatus, and the planar image will be referred to as an SLO image below.
In recent years, it has been possible to obtain a retinal SLO image with an improved horizontal resolution by increasing the diameter of the measuring beam in the SLO apparatus. However, there has been a problem in acquiring a retinal SLO image in that increasing the diameter of the measuring beam is accompanied by a decrease in the S/N ratio and in the resolution of the SLO image due to aberrations in the eye of the examination subject.
In order to resolve the above-mentioned problem, an adaptive optics SLO apparatus has been developed that has an adaptive optics system that measures aberrations in the eye of the examination subject in real-time using a wavefront sensor and corrects aberrations of a measuring beam or its return light that occur in the examination subject eye using a wavefront compensation device, thereby enabling the acquisition of an SLO image with a high horizontal resolution.
This SLO image having a high horizontal resolution can be acquired as a moving image, and in order to observe blood flow dynamics for example in a non-invasive manner, retinal blood vessels are extracted from the frames of the moving image, and the movement speed and the like of blood cells in capillaries are subsequently measured. Also, in order to evaluate the relationship between the photoreceptor cells and the visual function using the SLO image, photoreceptor cells P (as shown in FIG. 6B) are detected, and subsequently the density distribution and the alignment of the photoreceptor cells P are measured. FIG. 6B shows an example of an SLO image with a high horizontal resolution. The photoreceptor cells P, a low luminance region Q that corresponds to the position of a capillary, and a high-luminance region W that corresponds to the position of a leukocyte can be observed.
FIG. 6A shows an example of the various layers in the retina, from the inner limiting layer B1 to a pigmented layer B6. In the case of observing the photoreceptor cells P, measuring the distribution of photoreceptor cells P, or the like using the above-described SLO image, the focus position is set near the outer layer of the retina (B5 in FIG. 6A) and an SLO image such as FIG. 6B is captured. On the other hand, there are retinal blood vessels and bifurcated capillaries in the inner layers of the retina (B2 to B4 in FIG. 6A). 45% of the blood that is present in blood vessels is composed of blood cell components, and of those blood cell components, about 96% are erythrocytes and about 3% are leukocytes. An erythrocyte has a diameter of about 8 μm, and a neutrophil, which is the most common type of leukocyte, is about 12 to 15 μm in size.
As shown in FIG. 6C, if a leukocyte W is moving in a capillary in flow direction FD, small erythrocytes R flowing in the rear cannot pass the large leukocyte in the front, and therefore erythrocytes accumulate and an aggregation (hereinafter referred to as an “erythrocyte aggregate” DTi) forms behind the leukocyte. The size of the erythrocyte aggregate is at its smallest immediately subsequent to a vascular bifurcation (FIG. 6C), and it increases gradually as it nears the next vascular bifurcation (FIGS. 6D and 6E). Note that this aggregation occurs physiologically, and if the leukocyte is no longer in front of the erythrocyte aggregate, the erythrocytes will separate and move individually once again. If the focus position is set to the photoreceptor cells and an SLO image having a high horizontal resolution is acquired, the erythrocyte aggregate will be rendered as a dark tail behind the high-luminance leukocyte region W, as shown in FIG. 6F.
On the other hand, with a diabetic patient for example, erythrocytes will aggregate abnormally and form erythrocyte aggregates regardless of whether or not a leukocyte is present, as shown in FIG. 6G. Erythrocyte aggregates will be present at various positions in the capillary, including the position behind a leukocyte. Since the erythrocytes are constantly aggregated, the length of the erythrocyte aggregate behind the leukocyte will hardly change when moving between capillary bifurcations. Accordingly, by calculating the change in the size of the erythrocyte aggregate behind the leukocyte based on an SLO moving image with a high horizontal resolution, blood fluidity (the extent to which blood flows smoothly) can be measured in a non-invasive manner.
However, there has been a problem in that if the blood fluidity is to be measured, the capillary that is to be the target of analysis is selected manually, and therefore the procedure is cumbersome. Also, there has been a problem in that measurement values related to blood fluidity cannot be compared site-to-site. Therefore, a technique of    (i) automatically selecting an analysis target blood vessel in order to measure the blood fluidity of an eye area, and    (ii) displaying the distribution of the blood fluidity of the eye area is needed.
A conventional technique of generating a spatiotemporal image for a capillary branch region in an adaptive optics SLO moving image and measuring the degree of change in the length of the erythrocyte aggregate in the spatiotemporal image is disclosed in “Uji, Akihito, ‘Observation of dark tail in diabetic retinopathy using adaptive optics scanning laser ophthalmoscope’, Proceedings of the 66th Annual Congress of Japan Clinical Ophthalmology, p. 27 (2012)” as a technique for measuring blood fluidity in a non-invasive manner.
However, in the above-mentioned technique, the position of the capillary branch that is to be the position for measuring the length of the blood cell aggregate is specified manually, and there is a problem in that it is cumbersome to specify the capillary branch that is to be the measurement target from among the numerous capillary networks that are present in the parafovea.